The avian progesterone receptor is being used as a model for studying the role of phosphorylation in receptor structure and function. This receptor has been shown to be phosphorylated at multiple sites on serine residues. The present studies are designed to clearly described the extent to which the progesterone receptor is phosphorylated before and after progesterone treatment. Through peptide mapping experiments, the actual sites of phosphorylation will be determined within the primary structure of the receptor. There is now evidence for additional phosphorylation of the receptor following progesterone treatment in vivo. This will be described in detail as a receptor processing step that may relate to nuclear binding or regulation of gene expression. A cell-free system will be developed for studying this receptor processing which will allow the identification and characterization of all participating factors. These studies should provide a clear biochemical framework for assessing the significance of individual phosphorylation events to receptor activity.